LEDs are increasingly being used to replace lamps and bulbs in lighting applications including providing white light as aback light in color liquid crystal displays (LCD) and high definition televisions (HDTV). While LEDs may be used to uniformly light the entire display, performance, contrast, reliability, and power efficiency are improved by employing more than one string of LEDs and to drive each string to a different brightness corresponding to the portion of the display that the particular LED string illuminates. “Local dimming” refers to backlighting systems capable of such non-uniform backlight brightness. The power savings in such systems can be as high as 50% as compared with LCDs employing uniform backlighting. Using local dimming, LCD contrast ratios can approach those of plasma TVs.
To control the brightness and uniformity of the light emitted from each string of LEDs, special electronic driver circuitry must be employed to precisely control the LED current and voltage. For example, a string of “m” LEDs connected in series requires a voltage equal to approximately 3.1 to 3,5 (typically 3.3) times “m” to operate consistently. Supplying this requisite voltage to a LED string generally requires a step-up or step-down voltage converter and regulator called a DC-to-DC converter or switch-mode power supply (SMPS). When a number of LED strings are powered from a single SMPS, the output voltage of the power supply must exceed the highest voltage required by any of the strings of LEDs. Since the highest forward voltage required cannot be known a priori, the LED driver IC must be intelligent enough to dynamically adjust the power supply voltage using feedback.
In addition to providing the proper voltage to the LED strings, the backlight driver ID must precisely control the current conducted in each string to a tolerance of ±2%. Accurate current control is necessary because the brightness of an LED is proportional to the current flowing-it, and any substantial string-to-string current mismatch will be evident as a variation in the brightness of the LCD. Aside from controlling the current, local dimming requires precise pulse control of LED illumination, both in timing and duration, in order to synchronize the brightness of each backlight region, zone, or tile to the corresponding image in the LCD screen.
The prior art's solutions to the need for local dimming limit display brightness and are costly. For example, early attempts to integrate LED driver circuitry with multiple channels of high-voltage current sink transistors were problematic because mismatch in the forward voltage of the LED strings resulted in excessive power dissipation and overheating. Attempts to minimize power dissipation by lowering the current in the LEDs and limiting the number of LEDs in a. string (for better channel-to-channel voltage matching) proved uneconomical, requiring more LED strings and a greater number of channels of LED drive. Thus the full integration approach to LED backlight driver systems has been limited to small display panels or very expensive “high-end” HDTVs.
Subsequent attempts to reduce overall display backlight costs by using multichips approaches sacrificed necessary features, functionality, and even safety.
For example, the prior art multichip system for driving LEDs shown in FIG. 1 comprises backlight controller IC 6 which drives multiple discrete current sink transistors 4A-4Q and high-voltage protective devices 3A-3Q. The backlight comprises sixteen LED Strings 2A-2Q (collectively referenced LED strings 2). Each of LED strings 2A-2Q contains “m” series-connected LEDs. In practice, the number of LEDs in each string can range from two to sixty. Each LED string has its current controlled through one of discrete current sink MOSFETs 4A-4Q, respectively. Backlight controller IC 6 sets the current in each LED string in response to instructions from a backlight microcontroller μC 7, which are communicated through a high-speed, expensive, serial peripheral interface (SPI) bus 12. Microcontroller μC 7 receives video and image information from a scalar IC 8 in order to determine the proper lighting levels needed for each of LED strings 2A-2Q.
LED strings 2A-2Q are powered by a common LED power supply rail 11, which is biased at a voltage +VLED by switch-mode power supply (SMPS) 9. The voltage +VLED is generated in response to a current-sense feedback signal (CSFB) 10 from control IC 6. Supply voltages vary with the number of LEDs “m” connected in series and may range from 35 volts for strings of ten LEDs up to 150 volts for strings of 40 LEDs. Discrete protective devices 3A-3Q, typically high-voltage discrete MOSFETs, are optionally employed to clamp the maximum voltage present across the current sink transistors 4, especially for operation at higher voltages, e.g. over 100V.
In the system shown in FIG. 1 each component is a discrete device, in a separate package, requiring its only pick-place operation to position and mount it on its printed circuit board.
Each set of discrete components, along with the corresponding string of LEDs, is repeated “n” times for an “n” channel driver solution. For example, in addition to SMPS 9, the 16-channel backlight system shown in FIG. 1 requires 34 components, namely microcontroller 7, a high-pin-count backlight controller IC 6, 16 current sink transistors 4 and 16 protective devices 3, to facilitate local dimming in response to video information generated from scalar IC 8. This solution is complex and expensive.
Aside from requiring the assembly of a large number of discrete components, i.e. a high build of materials (BOM) count, the package cost of high pin count package 6 is substantial. The need for such a large number of pins is illustrated in the circuit diagram of FIG. 2, which illustrates in greater detail one of the channels of LED driver system shown in FIG. 1. As shown each channel includes a string 21 of “m” series-connected LEDs, a protective cascode-clamp MOSFET 22 with an integral high-voltage circuiting diode 23, a current sink MOSFET 24, and a current-sensing I-Precise gate driver circuit 25.
The active current sink MOSFET 24, implemented as a discrete component controlled by the interface IC 6, comprises a power MOSFET, preferably a vertical DMOSFET, having gate, source and drain connections. I-Precise gate driver circuit 25 senses the current in current sink MOSFET 24 and provides it with the requisite gate drive voltage to conduct a precise amount of current. In normal operation, current sink MOSFET 24 operates in its saturated mode of operation controlling a constant level of current independent of its drain-to-source voltage. As a result of the simultaneous presence of a source-drain voltage and current, power is dissipated in MOSFET 24. Continuous measurement of the drain voltage of current sink MOSFET 24 is required for two purposes—to detect shorted LEDs and to facilitate feedback to the switch mode power supply (SMPS) 9. The presence of a shorted LED is recorded in an LED fault circuit 27, and feedback to SMPS 9 is effected by a current sense feedback (CSFB) circuit 26.
In summary, current sink MOSFET 24 requires three connections to the control IC 6, specifically a source connection for current measurement, a gate connection for biasing the device to control its current, and a drain connection for fault and feedback sensing. These three connections per current sink MOSFET and hence per channel are depicted in FIG. 2A as crossing an interface 28 between the discrete devices and a control IC. Even in the schematic circuit diagram of FIG. 2B, where cascade clamp MOSFET 22 is eliminated and current sink MOSFET 24 must sustain high voltages, illustrated by “HV” integral diode 23, each channel still requires three pins per channel crossing interface 28. This 3-pin per channel requirement explains the need for high-pin count package 6 shown in FIG. 1. For a sixteen-channel driver, 3 pins per channel require 48 pins for the output pins on the control IC. Taking into account the SPI bus interface, analog functions, power supplies and more, a costly 64 or 72-pin package is necessary. Worse yet, many TV printed circuit board assembly firms are incapable of soldering packages with a pin pitch any smaller than 0.8 or 1.27 mm. A 72-pin package with a 0.8 mm pin pitch requires a 14×1.4 mm plastic body to provide the peripheral linear edge needed to fit all the pins.
One significant issue with multichip system shown in FIG. 1 is that temperature sensing in interface IC 6 can only detect the temperature of the IC, where no significant power dissipation is occurring. Unfortunately, the heat is being generated in the discrete current sink DMOSFETs 4, where no temperature sensing is possible. Without local temperature sensing, any of the current sink MOSFETs 4A-4Q could overheat without the system being able to detect or remedy the condition.
In summary, today's implementations for LED backlighting of LCD panels with local dimming capability suffer from numerous fundamental limitations with regard to cost, performance, features, and safety.
Highly integrated LED driver solutions require expensive large area dice packaged in expensive high pin count packages, and concentrate heat into a single package. This limits the driver to lower currents, due to power dissipation resulting from the linear operation of the current sink MOSFETs, and lower voltages, due to power dissipation resulting from LED forward-voltage mismatch, a problem that is exacerbated for greater numbers of series connected LEDs.
Multi-chip solutions combining an LED controller with discrete power MOSFETs require high BOM counts and even higher-pin-count packaging. Having nearly triple the pin count of fully integrated LED drivers, a sixteen channel solution can require 33 to 49 components and a 72 pin package as large as 14 mm×14 mm. Moreover, discrete MOSFETs offer no thermal sensing or protection against overheating.
What is needed is a cost-effective and reliable backlight system for TV's with local dimming. This requires a new semiconductor chip set that eliminates discrete MOSFETs, provides low overall package cost, minimizes the concentration of heat within any component, facilitates over-temperature detection and thermal protection, protects low voltage components from high voltages and against shorted LEDs, flexibly scales to accommodate a different number of channels and different sized displays without requiring custom integrated circuits, and maintains precise control of LED current and brightness.
Ideally, a flexible solution would be scalable to accommodate varying number of channels and display panels of different sizes without requiring custom integrated circuits.